Magnetic sensor systems are increasingly important in various industries. For instance in the automotive industry various sensor systems such as parking sensors, angular sensors e.g. in throttle valves, ABS (Automatic Braking System) sensors and tire pressure sensors can be found in modern vehicles for improving comfort and safety. Magnetic sensor systems are particularly important in automotive applications, because magnetic fields penetrate easily through most materials. Magnetic sensors are also highly insensitive to dirt, unlike for example optical sensors.
Several different magnetic sensor technologies are currently available, such as magneto transistors (MT), sensors based on the Hall Effect and sensors based on the magnetoresistive effect such as anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR) sensors. The sensing principle of AMR sensor systems is based on the physical phenomenon that the electric resistance of a ferromagnetic material depends on the angle between the magnetization and the direction of the electric current within an AMR sensing element. Hall sensors and MTs, which rely on the Lorenz force, have a comparatively low sensitivity and consequently also a low accuracy.
Known ABS applications are using AMR sensor systems in order to detect a magnetic field change of an encoder wheel being mechanically connected to a vehicle wheel or vehicle tire. A typical AMR sensor system comprises a Wheatstone full-bridge configuration built with magnetoresistive sensing elements (e.g. NiFe resistor stripes). In response to detected magnetic field changes caused by a rotation of the encoder wheel a speed-pulse information can be generated. Additionally, the recognition of the rotation direction of the encoder wheel may be possible by using the spatial difference of the two half-bridges of the Wheatstone full-bridge configuration. It is mentioned that encoder wheels can be realized as a magnetically passive or a magnetically active structure. A magnetically active encoder wheel comprises at least some portions with a permanent magnet. A magnetically passive encoder wheel, which is always used in connection with a biasing magnet, disturbs and/or changes the magnetic field which is generated by the biasing magnet and which is detected by the AMR sensor system. A rotating active or passive encoder wheel periodically changes the detected magnetic field.
Some known AMR sensor systems additionally have a reference bridge, which comprises resistive reference elements, which are not sensitive to an external magnetic field. However, such AMR sensor systems suffer from output signals which have only low amplitudes and which are typically very noisy.
More sophisticated AMR sensor systems comprise two Wheatstone full-bridge configurations each being built up with at least four AMR sensing elements. However, such AMR sensor systems are suffering from an additional effort in size, current consumption and costs.